Japanese Examined Patent Publication No. 2004-506394 (Patent Document 1) and Japanese Unexamined Patent Publication No. 2005-171763 (Patent Document 2) disclose acoustic sensors.
FIG. 1 is a plan view schematically illustrating a structure of a conventional acoustic sensor 11. In the acoustic sensor 11, a vibrating electrode plate 12 (movable electrode) and a counter electrode plate 13 (fixed electrode) face each other with a micro gap (air gap) interposed therebetween. The counter electrode plate 13 is exposed to the outside on a substrate, and the vibrating electrode plate 12 is covered with the counter electrode plate 13. In the counter electrode plate 13, plural acoustic perforations 14 (acoustic holes) are opened to have uniform opening areas and are disposed at equal intervals.
Because the vibrating electrode plate 12 is formed by a thin film, when a sound pressure (acoustic vibration) reaches the vibrating electrode plate 12 through the acoustic perforations 14 in the counter electrode plate 13, the vibrating electrode plate 12 vibrates microscopically in response to the vibration. A gap between the vibrating electrode plate 12 and the counter electrode plate 13 changes when the vibrating electrode plate 12 vibrates. Therefore, the acoustic vibration is detected by detecting a change in electrostatic capacitance between the vibrating electrode plate 12 and the counter electrode plate 13 with use of the acoustic sensor 11.
In the above acoustic sensor, the acoustic perforations are provided in the counter electrode plate, and the acoustic perforations have the following various functions in addition to the function of allowing the sound pressure to pass therethrough and vibrate the vibrating electrode plate:
(1) The sound pressure is released through the acoustic perforation such that the sound pressure is not applied to the counter electrode plate.
(2) Air in the micro gap is released through the acoustic perforation, thereby preventing air damping of the vibrating electrode plate.
(3) The acoustic perforation acts as an etching hole when the micro gap is prepared between the vibrating electrode plate and the counter electrode plate by utilizing the micromachining (semiconductor microfabrication) technique.    Patent Document 1: Japanese Examined Patent Publication No. 2004-506394    Patent Document 2: Japanese Unexamined Patent Publication No. 2005-171763    Patent Document 3: Japanese Unexamined Patent Publication No. 2004-128957